1. Field of the Invention
This invention relates to a method of preparing coupler dispersions for photographic use and more particularly to a method of preparing coupler dispersions being substantially free of auxiliary coupler solvents by a membrane separation technique.
2. Description of Related Art
In the manufacture of film dispersions a photographic coupler is dissolved in a permanent coupler solvent with the addition of an auxiliary coupler solvent that assists in the dissolution of the coupler in the permanent coupler solvent. This solution is mixed under high shear, together with an aqueous gelatin solution generally containing a surfactant, at elevated temperatures in order to break the organic phase into sub-micron droplets dispersed in the continuous aqueous phase.
Subsequently, the dispersion is chilled and extruded into "noodles" which are approximately three millimeters in diameter. These noodles are washed for several hours in an abundance of water with agitation to extract the auxiliary coupler solvent. The noodles are drained over night to reduce the water content. The entire process takes on the order of one day, is labor intensive and is inefficient due to coupler loss which occurs mainly during the washing process. Various aspects of this noodling procedure are disclosed in the following U.S. Pat. Nos. 2,322,027; 2,801,170; 2,801,171; 2,949,360; and 3,396,027. Another disadvantage of the noodling procedure, that is also mentioned in several of the above-mentioned patents, is that the coupler has a tendency to crystallize in the emulsion upon the removal of the auxiliary coupler solvent. This has associated disadvantages in that the coupler reacts less readily in the color forming reaction, this being the prime function in the photographic element.
U.S. Pat. No. 4,233,397 removes the auxiliary solvent from a coupler dispersion by contacting the coupler dispersion containing the auxiliary solvent through a hydrophobic macroporous film made of polytetrafluorethylene or polypropylene with an auxiliary solvent-carrying fluid medium. The auxiliary solvent-carrying fluid medium is water, air and non-reactive gases saturated with water and aqueous solutions of compounds that increase the solubility of the auxiliary solvent in water, such as glycerin, ethylene glycol, formamide and dimethylformamide. The hydrophobic membrane has an average pore size of about 0.1 to 40 micrometers preferably from 0.1 to 5 micrometers.